The invention relates to processes for cleaning carbon dioxide-containing process gases from the preparation of vinyl acetate after reaction of ethylene with acetic acid and oxygen in heterogeneously catalyzed continuous gas-phase processes.
Vinyl acetate is an important monomer building block for the production of polymers such as, for example, vinyl acetate-homopolymers or vinyl acetate-copolymers with ethylene, vinyl chloride, acrylates, maleates, fumarates or vinyl laurate. Vinyl acetate is prepared conventionally in a fixed-bed tubular reactor (or else fluidized-bed reactor) in an exothermic reaction of ethylene with acetic acid and oxygen in heterogeneously catalyzed continuous gas-phase processes. In this case, usually fixed-bed catalysts are used and generally contain palladium salts and alkali metal salts on a support material and in addition can be further doped with gold, rhodium or cadmium. The reaction is usually carried out at a pressure from 1 to 30 bar and a temperature from 130° C. to 200° C.:C2H4+CH3COOH+½O2→CH3COOCH=CH2+H2O.
In a side reaction, ethylene is oxidized to CO2 (carbon dioxide):C2H4+3O2→2CO2+2H2O.
The greater is the fraction of ethylene which is reacted to form byproducts, the lower necessarily is the selectivity of the reaction of ethylene to form vinyl acetate. Ethylene selectivities that are as high as possible are therefore desirable.
The gas mixture fed to the reactor generally contains a several fold molar excess of ethylene. The reaction of ethylene and the further reagents in the reactor proceeds incompletely. The process gas (product gas stream) taken off from the reactor contains substantially vinyl acetate, ethylene, acetic acid, water, oxygen and also byproducts, predominantly carbon dioxide, and the inerts nitrogen, argon, methane and ethane. From the product gas stream, after it leaves the reactor, the reaction product vinyl acetate, unreacted acetic acid, water and further condensable fractions are condensed out as far as possible and fed to further workup and purification by distillation up to isolation of the pure vinyl acetate. The gas stream remaining after separating off vinyl acetate and condensable fractions from the product gas stream is finally recirculated back to the reactor as circuit gas after further purification steps.
Sufficient ejection of the byproduct carbon dioxide is worthy of particular attention in the purification of the circuit gas. Otherwise, during the continuous operation of the process, an enrichment of carbon dioxide in the circuit gas occurs, as a result of which the vinyl acetate formation in the reactor would be inhibited within a short time. For this reason, in current processes, a part of the circuit gas is branched off before it is recirculated to the reactor and fed to the CO2 scrubber (CO2 absorption/desorption) to separate off CO2 and then combined with the further circuit gas.
The circuit gas is a gas mixture consisting of predominant fractions of ethylene, carbon dioxide, ethane, nitrogen and oxygen. The circuit gas, before recirculation into the fixed-bed tubular reactor, is admixed with the reactants acetic acid, ethylene and oxygen and brought to reaction temperature using heat exchangers operated by heating steam. To compensate for pressure losses, the circuit gas is generally compressed by means of a circuit gas compressor before recirculation to the reactor.
Such processes are known, for example, from DE-A1 102009002666. DE-A 102006038689 describes processes for isolating vinyl acetate from the abovementioned product gas stream, wherein the steam expelled in the CO2 desorber is used energetically. Also EP-A1 1760065 describes processes for isolating vinyl acetate from the product gas stream, wherein the acetic acid produced in the circuit gas scrubber is recycled in a defined manner in the process for vinyl acetate preparation.
Against the background of these developments, also, there continues to be a need to configure the separation of carbon dioxide from the circuit gas to be more efficient, in particular less energy-intensive.
The CO2 scrubber of the circuit gas has been operated for many decades generally using aqueous potash solutions. For improved separation of carbon dioxide from gases, in recent years also in connection with the storage of carbon dioxide, also termed Carbon Capture and Storage (CCS), a number of relatively efficient absorption media or additives have been developed such as, for example, methylamine or ethanolamine. Such new developments from other technologies, however, are not simply transferrable to processes for cleaning circuit gas of vinyl acetate preparation, since in this case interventions are made into a complex gas mixture, whose composition has been further optimized for decades. It would also be fatal, if, by modifying the absorption medium, impurities were to be introduced into the circuit gas and further utilization or reaction of the circuit gas would be impaired.